A large portion of the world's electric power is generated from burning fossil fuels such as coal. The four primary types of coal (ranked from high to low) are anthracite, bituminous, sub-bituminous and lignite. Higher-rank coals typically contain less moisture and fewer pollutants than lower-rank coals. Coal is typically dried to enhance its rank and heating value (kJ, BTU per pound). In addition to enhancing rank and heating values, drying coal provides additional benefits. For example, once moisture has been removed after drying, coal is lighter and can be transported more easily and with less expense. Thus, coal drying is an important step in electric power generation.
Various coal drying methods and systems have been used in the past several decades including rotary kilns, cascading whirling bed dryers, elongated slot dryers, hopper dryers, traveling bed dryers and vibrating fluidized bed dryers. Many of these methods and systems require high temperatures and pressures. Because large amounts of energy are needed to reach these high temperatures and pressures, drying lower-rank coals with these methods can be economically impractical. Thus, efforts have been made to develop coal drying methods using lower temperatures and pressures. Many low temperature methods utilize fluidized bed technology, but are able to dry coal only to a limited extent. Subsequent high temperature steps are sometimes used to further dry coal processed at low temperatures. One issue encountered with fluidized bed drying of coal is the production of fines that become entrained in the fluidizing medium. In an environment where oxygen, and in some cases the ignition energy, is readily available, these fines can spontaneously combust. Thus, these drying methods typically use inert fluidizing gases such as nitrogen, carbon dioxide and steam to provide an environment with limited oxygen in order to prevent combustion.
Efforts have also been made to increase the efficiency of coal drying systems by using waste heat streams as heat sources. Waste heat streams include coke cooling gas, flue gas, stack gas, and steam condensate from power generation turbines. One or more waste heat streams can be used alone to provide heat to coal drying systems or in conjunction with primary heat sources, typically provided by the combustion of fossil fuels.
While past innovation has provided advancement of coal drying techniques, further improvements in coal drying efficiency and cost are desired. Even small improvements in coal drying efficiency can have huge, beneficial effects. A five percent increase in efficiency can mean tens of millions of dollars in savings per year for an average size power plant.